Video recordings of surveillance systems are typically carried out 24 hours a day, 7 days a week, 365 days a year. This may require the use of mass storage capacity and managing capabilities to gain the most efficient and reliable recording process. Organizations that utilize large numbers of surveillance cameras, such as casinos, airports, large banks or secured government institutions, are required to handle thousand of hours of real time video recordings.
Reliability of video recording is a critical issue for surveillance systems. Video recordings of surveillance systems may be used for the investigation of various events and in some situation may be used in the court of law as evidence. Thus, the video recording should be continuous with no substantial gaps that may disable reliable interpretation of the video.
It is known in the field of streaming that networks are prone to packet loss. According to Real-time Transport Protocol (RTP) the header of the packets includes a sequential number so that the receiver side would be able to detect packet loss. RTP is regarded as the primary standard for audio/video streaming in IP networks, however, the protocol does not specify any action on packet loss, or how packet loss is detected at the receiver side. For example, video steaming applications that do not require high reliability may allow some level of packet loss which may be considered negligible. This level of packet loss is unacceptable for applications that require high reliability of video streaming, such as surveillance systems.
Transmission Control Protocol (TCP) on the other hand, includes mechanisms to ensure that all of the packets would arrive to the receiver side by retransmitting lost packets. However, retransmission involves halting the sending of the next packets until the missing packets are received. This interruption in the transmission is not acceptable for real time streaming. User Datagram Protocol (UDP), is often selected for real-time systems that do not require high reliability, however, due to its real-time and low delay nature, it does not handle packet loss.
Video recordings of surveillance systems are typically performed using state of the art block based video compression algorithms such as H.263, Moving Picture Experts Group (MPEG) 2, H.264/MPEG-4 Advanced Video Coding (AVC), etc. Video block based compression algorithms typically generate compressed frames which act as reference frame to the next frame. Intra frames are frames that are compressed independently of other frames and therefore may be decoded independently. A GOP (group of pictures) is defined from an intra frame to the next intra frame in a stream. Inter frames, on the other hand, depend on previous frames (P-frames), either the previous intra or inter frames, and sometimes on both previous and next frames (B-frame), for compression at the encoder side and for decoding at the decoder side.